1. Field of the Invention
The present invention relates to an information storage medium capable of reproducing information recorded thereon as a mark with a size less than the resolution limit of a reproduction beam and having an improved carrier-to-noise (C/N) ratio, a C/N improving method and a data recording and/or reproducing apparatus.
2. Description of the Related Art
An optical recording medium is used as an information storage medium used in an optical pickup apparatus which performs recording and reproducing information contactlessly. With the ongoing industrial development, a higher recording density of information is desirable. For this, an optical recording medium capable of using a super resolution phenomenon in which the medium has a recording mark with a size less than the resolution limit of a laser beam is under development.
Information storage media are classified into reproduction dedicated storage media (read only memory, ROM) only for reproducing recorded information, write-once-read-many storage media capable of recording data only once, and rewritable storage media capable of recording, deleting, and rewriting data.
One of the most important characteristics of the information storage media is the storage capacity. The increase in the capacity of a storage medium depends on how small a mark can be recorded in a predetermined area of a storage medium, and how accurately the recorded mark can be reproduced.
In particular, the information reproduction performance depends on shortening the wavelength of a light source used in reproducing information, or increasing the numerical aperture of an object lens. However, with conventional technologies, there is a limit in providing a laser having a short wavelength, and in order to manufacture an object lens with a high numerical aperture, there is a limit in that the manufacturing cost is very expensive. In addition, as the numerical aperture of an object lens increases, the working distance between an optical pickup and a storage medium decreases and the possibility that information recorded on the storage medium is damaged by a clash between the optical pickup and the storage medium increases. For these and/or other reasons, it is not easy to implement a high capacity and high density storage medium.
Furthermore, when the wavelength of a light source to reproduce information on a storage medium is λ and the numerical aperture of an object lens is NA, the minimum value of the reproduction resolution is λ/4NA. Accordingly, though it is possible to form a recording mark with an extremely small size, the reproduction can be impossible. That is, since the light irradiated from the light source cannot distinguish a recording mark with a size less than λ/4NA, it is generally impossible to reproduce the information.
Meanwhile, there is a super resolution phenomenon in which a recording mark with a size less than the limit of resolution power is reproduced, and the analysis and research and development of this super resolution phenomenon are briskly proceeding. According to the super resolution phenomenon, reproduction of a recording mark with a size less than the resolution limit is also possible such that the super resolution recording medium can dramatically satisfy the demands for the high density and high capacity.
There are a variety of basic recording characteristics and reproducing characteristics. Among them, the most important thing is to secure a good carrier-to-noise ratio (hereinafter, referred to as ‘C/N’).
In particular, since a super resolution information storage medium uses a recording beam and reproducing beam with a power relatively higher than that of an ordinary information storage medium, the improvement of the C/N ratio becomes very important.